The present invention relates to a device for phase modulation of coherent light with a spatial light modulator which comprises a modulator matrix with a regular arrangement of controllable LC modulator cells, with at least one light source which illuminates the modulator matrix, and with a control unit which controls the phase modulation in the LC modulator cells. A three-dimensional representation of images can be realised with this device if it is used as a display device.
The field of application of this invention includes coherent-optical applications which comprise for modulating coherent light a liquid crystal (LC) device which is used to control the phase of the light in phase steps, whose number typically ranges between more than fifty and several hundreds, in a modulation element of the LC device. The LC device can be used for three-dimensional, preferably holographic image representation.
In the field of coherent-optical applications there is a demand for a very fast switching spatial light modulator (SLM), with switching delays in a range of less than 1 ms, which can preferably be used as a phase-modulating spatial light modulator. Liquid crystal (LC) modulators (LC SLM) can be used for this, which have the advantage over other modulator types that they can be manufactured inexpensively with conventional production technologies.
The switching delay of the LC molecules in a LC SLM depends among other factors on the LC type (nematic, smectic, . . . ) and on the arrangement of the LC molecules in a modulator cell of the modulator matrix of the LC SLM.
The publication [1], A. Mochizuki, Journal of the SID 14 (2006) pp. 529-536, discloses the idea of using a polarisation shielded smectic liquid-crystal-type spatial light modulator (PSS LC SLM) as an amplitude-modulating light modulator. Switching delays of less than 500 μs are being described for this type of modulator. The long axes of the LC molecules are described to generally move, when an electric field is applied, along the shell of a cone, but where the cone is compressed. This compression deforms the base of the cone such that it turns from a circle into a narrow ellipse. In the present case, the major axis of the ellipse lies in the screen plane, and the LC molecules move mainly in the screen plane.
Document US 2007/0003709 also describes the design of a PSS LCD used as an amplitude-modulating light modulator.
Further, it is known that for controlling SLM with nematic LC the sign of the voltage is altered in every other frame, which is known as ‘frame inversion’. This is done to prevent both the occurrence of undesired chemical processes, which would otherwise take place when a DC field is applied, and image sticking. Frame inversion is realised in one embodiment such that in a first frame all modulator cells of the modulator matrix are controlled with a positive voltage and in a second frame all modulator cells of the modulator matrix are controlled with a negative voltage.
Alternatively, the sign of the voltage can be altered dot-wise (‘dot inversion’) or line-wise (‘line inversion’) in order to prevent flickering. With line inversion, in a first frame even columns of the modulator matrix are controlled with a positive voltage and odd columns are controlled with a negative voltage, for example. In a second frame, the columns are controlled vice versa. With dot inversion, the modulator matrix shows a chessboard-like pattern of positive and negative control voltages.
In conventional nematic LC SLM, the orientation of the LC molecules depends only on the absolute value of the applied voltage, not on the sign. The orientation of these LC molecules does thus not change when frame inversion is employed.
In contrast, publication [1] discloses that in LC SLM of the PSS liquid crystal type (PSS LC) there are two different, mirror-symmetrical orientations of the LC molecules when the sign of the voltage which is applied to the LC layer changes. However, these two orientations effect the same amplitude of the modulated light, so that a PSS LC which is used as an amplitude-modulating light modulator can be controlled using frame inversion, line inversion or dot inversion methods, just like a conventional modulator with nematic LC.
Further, document [2], S. Pancharatnam, Proc. Ind. Acad. Sci. 41 (1955) pp. 130 (Parts I and II), discloses an arrangement of a λ/2 plate which is disposed between two λ/4 plates, and which is used as a phase-modulating light modulator, as the optical axis of the λ/2 plate is controlled in different angles in relation to the λ/4 plates. Further, [2] suggests to realise achromatic λ/4 plates as a combination of multiple birefringent layers, and to obtain an achromatic circular polariser by combining two λ/2 plates and one λ/4 plate, which are arranged under certain angles of their optical axes.
Such a configuration is also described in document [3], G. Love, R. Bandari, Opt. Commun. 110, (1994), pp. 475-478, for a ferro-electric LC SLM, which switches very fast, but which has the disadvantage that only two phase conditions can be realised per modulator cell. In other words, it is a binary LC SLM.
A similar configuration is also possible for an in-plane switching (IPS) LC SLM, which is used as a controllable λ/2 plate between two fix, non-controllable λ/4 plates. A continuous phase modulation between 0 and 2π is possible by turning the LC molecules by an angle ranging between 0 and 90 degrees in the display plane by applying an electric field.
In contrast to a commonly used dual-domain IPS LC SLM, where a modulator cell is divided into two halves in order to realise an enlarged viewing angle range, a single-domain structure with a homogeneous modulator cell would be necessary to get a phase-modulating light modulator. However, today's IPS LC SLM do not achieve the desired switching delays of less than 1 millisecond. Simply modifying an IPS LC SLM would therefore not be adequate to get a fast switching phase-modulating light modulator.